A super simple thing that I was surprised wasn't already here.

Spool clamps are used by luthiers to build violins and other stringed instruments. Typically they're made from a hardwood dowel with a threaded rod trilled through the center, with cork or leather lining the jaws. They're great for clamping the edge of things without having to worry about the orientation of the clamp.

They cost about $4 each at a luthier supplier, which doesn't seem too much but you really need a couple dozen of them for any real project, and that starts to add up quickly. Everyone says that it isn't any cheaper to make your own, but I figure plywood disks and carriage bolts are cheaper than that. And I have a project I can use them on.

Two 1" disks. One has a 0.26" hole in it for a 1/4" bolt. The other has a 0.26" square to fit the square end of a carriage bolt. I'm providing the OpenSCAD script and DXF files for each disk.

You need a 1/4" carriage bolt, a 1/4" fender washer, and a wing nut. You can use a regular bolt, but I'd suggest you use another washer on the other end.

I used 1/4" plywood, with 1 square-hole disk, 5 round hole disks, and two round hole disks cut from 1/8" cork. The other is:

square bit of carriage bolt
square hole disk
round hole disk
round hole disk
cork
<the thing you're clamping goes here - no glue>
cork
round hole disk
round hole disk
round hole disk
washer
wing nut

Put a bit of glue between each of the wood/cork layers as you put them on the bolt. It clamps itself :)

The glue is necessary - if you don't use it, the jaws of the clamp won't stay perpendicular.

You can probably get away with two disks on each side of the clamp. I used three because that's close to what the traditional clamps look like.